Well, if you think that moving an app to the trash gets rid of it entirely, you're mistaken. Many files often get left behind, from preference files (harmless really) to system services and/or preference pains (a lot more troublesome if the app isn't around). And, what about those pkg installers? You know, the ones that throw files all over the place? There's no real simple way to get rid of an app you've installed by that method and, worse, they're should be as it wouldn't take much work for Apple to implement a system that monitors everything a pkg does and would allow it to be undone.
Apple certainly got the installation process about as close to perfect as it's ever likely to get on an open system, I'm not disputing that. But the process of removing an app on OS X is absolutely painful, and I say that as a full-time OS X user and a mostly satisfied one at that. It's an area that Apple really should focus their efforts on solving, not that most uninstallers on Windows do a much better job of removing an app than just dragging it to the trash does. *NIX is the only system that really has removing software down pat, but then again that's only for software in the repos or ports tree (or otherwise controlled by the package manager) (anything else and you're on your own worse than in OS X).